The present invention generally relates to plastic forming processes (casting, molding, etc.) and, particularly, to extruders used in such processes. More particularly, the present invention relates to an apparatus for removing screw elements from an extruder screw shaft for cleaning, inspection, repairs or replacement.
Extruders are used extensively in the plastics industry. For example, extruders are typically employed in the plastics industry for producing plastic pellets from raw materials. These extruders comprise of two basic sections: the process section (extruder screw and barrel) and the forming section (head and die).
Raw material is loaded into a hopper which gravity feeds to an extruder screw rotatably mounted in a heated barrel. The screw is the heart of the extruder and consists of feed, transition and metering zones. The feed zone conveys the solid or sometimes half-molten or molten raw material out of the feed throat area to the transition zone, which starts compressing the preheated material. This zone forces the plastic against the heated barrel and continues the melting process, which is completed at the beginning of the metering zone. The metering zone conveys the molten plastic to the head and die section at uniform rates and under high pressure. The molten plastic passes through the die, where it is continuously formed into round strands or filaments (or other geometrical shapes). The round strands are then carried through a water cooling bath. When sufficiently cooled, the strands are fed to a chopper to be cut into pellets.
These pellets are produced in many different colors depending upon the application. Typically, injection molding process is used to transform these plastic pellets into molded products at high production rates and with good dimensional accuracy. For example, injection molding machines are used for molding television cabinets in assorted colors in the consumer electronics industry.
A conventional extruder screw comprises a rotatable shaft carrying a plurality of screw elements, which are configured to form a continuous helical spiral or thread on the outer surface thereof Each of these screw elements have a central opening therein for receiving the shaft. The outer diameter of the shaft is slightly smaller than the inside diameter of the opening in the screw elements to provide a slip fit. The screw elements are slid over the shaft from one end ("the downstream end") and securely held in place against a collar or a flange either secured or integrally formed at the other end ("the upstream end"). The inner surfaces of the opening in the screw elements and the outer surfaces of the shaft are splined or serrated to ensure a good coupling between the screw elements and the shaft.
Normal extrusion operations at high temperatures and pressures force molten plastic between the screw elements and the shaft and into the splines meshing the screw elements to the shaft. Often the screw elements bond to the shaft due to close tolerances, high temperatures and use of highly filled plastics.
It is desirable to remove the screw elements from the screw shaft every time there is change in the pellet color to avoid cross contamination (which can be often depending on the batch size). The screw elements can then be thoroughly cleaned to remove the plastic from the previous batch and re-installed on the screw shaft. The need for frequent removal of the screw elements from the shaft is a problem that has plagued the extruder industry for a long time.
Screw elements may be heated to very high temperatures (for example, 900 degrees Fahrenheit) and forced out by using a sledge hammer and a brass bar (so-called "heat and beat" technique). This method of removing the screw elements from the shaft is undesirable due to the damage it can cause to the microstructure and the dimensional accuracy of the screw elements and the shaft. The hardened and wear resistant screw shaft and elements are particularly susceptible to cracking. Local overheating can cause deformation of the screw elements. The screw element extractor in accordance with the present invention allows removal of the screw elements from the screw shaft while reducing the risk of element damage, reducing the time for element removal and improving the safety of the personnel.
According to a first aspect of the present invention, an apparatus for extracting screw elements from a screw shaft comprises a frame, a stationary clamp mounted on the frame and disposable between open and closed positions, a carriage mounted on the frame for motion toward and away from the stationary clamp, a movable clamp mounted on the carriage for rectilinear motion therewith and disposable between open and closed positions, a first drive coupled to the stationary clamp for opening and closing the stationary clamp, a second drive coupled to the movable clamp for opening and closing the movable clamp, and a third drive coupled to the carriage for causing motion of the carriage toward and away from the stationary clamp, wherein clamping of a downstream screw element by the stationary clamp, clamping of an upstream screw element (or an upstream section of the screw shaft) by the movable clamp and one or more strokes of the carriage away from the stationary clamp result in removal of the downstream screw element from the screw shaft. The procedure can be repeated to remove all the remaining screw elements.
In accordance with another aspect of the present invention, a screw element extractor and a method of operating a screw element extractor comprise an apparatus for, and the steps of, mounting a stationary clamp on a frame, reciprocably mounting a carriage on the frame for motion toward and away from the stationary clamp, mounting a movable clamp on the carriage for motion therewith, opening the stationary clamp, opening the movable clamp, positioning the carriage adjacent the stationary clamp, supporting a screw shaft along an axis defined by the stationary and movable clamps such that its downstream end is disposed toward the stationary clamp and its upstream end is disposed toward the movable clamp, positioning a downstream screw element into the stationary clamp, closing the stationary clamp to hold the downstream screw element in place, closing the movable clamp to clamp an upstream screw element (or an upstream section of the screw shaft), moving the carriage away from the stationary clamp to extract the downstream screw element held in place by the stationary clamp, and opening the stationary clamp to release the downstream screw element.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived and the accompanying drawings which illustrate the invention.